1. Field of the Invention
The present invention relates to a laminated film comprising at least two thin films of LnBa.sub.2 Cu.sub.3 O.sub.7-x wherein Ln is Y or one of lanthanoids except Pr and Tb which attracts attentions as a superconductor exhibiting superconductivity around 90 K and at least one thin film of Y.sub.2 O.sub.3 which is interposed between a pair of the thin films of LnBa.sub.2 Cu.sub.3 O.sub.7-x, and a method for producing such laminated film.
Such laminated film will find various applications such as Josephson tunneling junctions or other electronic devices.
2. Description of the Related Art
A thin film of a single crystal of superconductive LnA.sub.2 Cu.sub.3 O.sub.7-x wherein Ln is the same as defined above and A is at least on alkaline earth element having the (001) plane in the direction parallel with the film surface and its preparation are disclosed and claimed in U.S. Patent application Ser. No. 247,261 filed on Sep. 16, 1988 or EP-A 0 308 869.
It is contemplated to apply the above method in the preparation of a laminated film comprising a pair of thin films of LnA.sub.2 Cu.sub.3 O.sub.7-x superconductor and an insulating thin film which is interposed between the pair of the superconductor films. For example, on a suitable substrate, a thin film of a single crystal of LnA.sub.2 Cu.sub.3 O.sub.7-x superconductor having the (001) plane in the direction parallel with the film surface is formed by the above method, then a suitable insulating thin film is formed on the LnA.sub.2 Cu.sub.3 O.sub.7-x thin film, and finally a second thin film of LnA.sub.2 Cu.sub.3 O.sub.7-x superconductor is formed on the insulating thin film by the above method.
However, it is not sure whether or not a laminated film which can be used in the electronic devices is produced. In particular, according to a kind of the insulating thin film or a method for forming the insulating thin film, it is difficult to predict how the LnA.sub.2 Cu.sub.3 O.sub.7-x thin film is influenced.
When the laminated film is used as an electronic device, it is important that a pair of the LnA.sub.2 Cu.sub.3 O.sub.7-x thin films between which the insulating thin film is interposed have good superconductive characteristics.
To form a Josephson tunnel junction, an insulating layer should have a thickness of 50 .ANG. or less since a high temperature superconductor has a very short coherence length. In other electronic devices, the insulating layer preferably has a thickness of 100 .ANG. or less. Though it is necessary for the superconductor layers to be completely insulated on the both sides of the insulating layer and no short circuit should be formed. Hitherto, it has not been reported that such insulating layer for the superconductive thin layers could have been formed.
In general, in an initial stage of the thin film formation, a material has a structure like islands in a sea, and the thin film should have a thickness of 100 .ANG. or larger to form a continuous film. In the case of epitaxial growth, it is preferable to provide an oxide which forms a continuous film at a thickness of several ten .ANG.. However, no oxide is known that can be grown in such manner on LnBa.sub.2 Cu.sub.3 O.sub.7-x, or no theory for predicting the presence of such oxide is present.